1. Field of the Invention
The invention relates to fabric stretchers. More particularly, this invention relates to an apparatus for stretching screen fabric for use in silk screen processing or the like.
2. Description of the Related Art
Stretching frames are used for a wide variety of applications. One prevalent use for such frames is in the field of graphic arts. In the medium of oil painting, the canvas must be stretched into a taut plane so that paint may be applied onto a firm and stationary surface. Frequently the frame used for such art is made of wood and the canvas is attached to one side and then pulled across to the opposite side where it is anchored. Such frames may be fashioned so that a wedge may be driven into each of the corners of the frame to provide tension for tightening the canvas.
Silk screens may be used for transfer of inks for the creation of an artwork or in the printing industry. Screen mesh in current use in the screen print industry is manufactured from polyester, or in some cases stainless steel fabrics. The polyester or stainless steel materials are typically supported by and tensioned between opposing sides of a four sided frame. Screen mesh, when stretched into a taut plane, provides the foundation for a stencil. Printing ink can then be forced through the open areas of the stencil by means of a flexible blade or squeegee, creating an image in the shape of the stencil on a workpiece positioned beneath the screen.
High quality, high definition screen printing depends on several factors if optimum results are to be achieved. The screen must be maintained at the recommended working tension throughout the print run so that the pattern is transferred with accuracy. Tension variation across the screen mesh must be kept to a minimum.
A specific and dimensionally constant "off contact" distance must be maintained between the screen mesh and the workpiece to minimize ink build up on the underside of the screen and to prevent rubbing contact between screen and workpiece. During the printing stroke the squeegee blade deflects the fabric downwards forcing it into direct contact with the workpiece during its passage across the screen. This deflection causes the fabric to be stretched from its undisturbed condition. The constant cycle of stretching and "snapping back" accelerates the deterioration of both screen and stencil. Additionally, as the screen mesh undergoes these distorting forces during the print stroke, the stencil on the screen may also be distorted, especially if the off contact is high. This causes a slightly distorted image to be reproduced on the workpiece.
In order to tension the fabric on a tubular alloy frame, the frame tubes must be rotated in opposition to each other with the mesh locked into the frame tubes. The resulting rotational moment often causes the frame to distort slightly in the horizontal plane when tensioned. This distortion causes the off contact distance between screen and workpiece to vary across the print area with a consequent reduction in print quality. The same applies to wood frames where the frame bars tend to warp in service due to changes in temperature and humidity. A sliding method of providing screen tension will ensure that the screen mesh remain in the horizontal plane across its entire surface because of the absence of residual twisting forces.
Popular methods of loading the fabric behind the frames' locking strips introduces the probability of tension variations. This occurs because the strip is forced along a narrow channel along the length of the frame bar with the fabric trapped beneath it in the slot. The mesh may roll up ahead of the leading edge of the strip causing loose pockets of fabric to develop, with consequent tension variations occurring in the fully tensioned screen. Another problem occurs when the screen is held at discrete places on the frame, whereby the fabric of the screen is not stretched uniformly all along the edge of each side of the frame, again causing tension variations.
Screen frames currently available do not allow the mesh and stencil to be easily removed and reinstalled on the frame. Consequently screens must be stored mounted in their frames between print runs. Removal of the mesh and accurate reinstallation for later use is a major improvement over current technology since the cost of frames is high and the reuse of screens is quite common. Additionally, storage of only the screen mesh uses a small fraction of the space required for storing a complete screen and frame assembly.
Currently known frame structures and framing methods, including those available for commercial silk-screen processing suffer from one or more of the foregoing drawbacks. The presently disclosed invention teaches a frame that overcomes the drawbacks of the prior art.